Research on NVH analysis and optimization of electric drive assembly of pure electric vehicles

With the development of the pure electric vehicle industry, the integration level of electric drive assemblies is getting higher and higher, and domestic parts manufacturers have launched "two-in-one", "three-in-one" and "six-in-one" drive system assemblies. While providing vehicle customers with fast and convenient power matching, the problems that have always existed in the electric drive system and the reasons for their occurrence are becoming more and more complicated, including NVH, efficiency and comprehensive durability of the powertrain.

Analysis of the causes of electric drive assembly howling

The electric drive assembly of a pure electric vehicle usually consists of a motor and a reducer, and most of them use a combination of a permanent magnet synchronous motor and a two-stage reducer. The reasons for the howling of the electric drive assembly are complex, mainly including: electromagnetic excitation of the motor, resonance of the reducer system, and coupled modal resonance of the electric drive assembly system. During the vehicle test of a certain model of electric drive assembly, the customer found that there was a structural resonance problem. This article mainly uses MASTA software analysis to simulate and analyze the powertrain, find out the cause of the structural resonance of the powertrain, and correct it.

During the NVH test of the whole vehicle, the near-field noise data in the vehicle can be collected through the LMS data acquisition front end, and the collected data can be used to perform noise order analysis on the near-field noise through the LMS Test.Lab data analysis software to find the corresponding order of the howling noise. Then, the excitation source of the howling noise can be determined through the howling noise order analysis.

Figure 1 Waterfall diagram of the noise inside a certain model of drive assembly

Figure 2 22nd order noise diagram

The test data of NVH test of a certain model of electric drive assembly in this paper is shown in Figure 1. According to customer feedback, under the WOT condition of the whole vehicle, when the input speed is between 1600 and 2000 r/min (586.6 and 733.3 Hz), there is a resonance whine problem in the 22nd order of the electric drive assembly. According to the structure of the electric drive assembly, it can be basically determined that the noise is generated by the high-speed stage of the reducer in the drive assembly.

As shown in Figure 2, the 22nd-order noise of the assembly has an obvious mutation at around 2,000 r/min; as shown in Figure 1, except for the 22nd-order, the system resonance response of other orders of noise near 696 Hz is obvious. It can be judged that the assembly has system structural resonance near 696 Hz, and the system structure needs to be adjusted to improve this situation.

Analysis of reducer assembly order noise

1. Establishment of MASTA reducer analysis model

The MASTA analysis model is established according to the electric drive assembly product, as shown in Figure 3. The electric drive assembly gear parameters are shown in Table 1.

2. MASTA software analysis system model

After inputting the macroscopic parameters and microscopic modification of the gear pair into the software, the transmission error of the high-speed gear pair of the electric drive assembly under the WOT condition of the whole vehicle is obtained through MASTA software simulation, as shown in Figure 4. The Fourier transform result of the transmission error of the high-speed gear pair is shown in Figure 5. Under the WOT condition of the whole vehicle, the peak calculation result of the transmission error of the high-speed gear pair meets the design requirements, and the frequency domain amplitude of the transmission error is not large, indicating that the 22nd order excitation of the drive assembly is not actually large, and the cause of the howling problem should be the existence of system resonance.

The system coupling mode is analyzed, and the results are shown in Figures 6 and 7. In the range of 1 000 to 2 000 r/min, there are multiple potential resonance points between the 22nd order excitation and the system natural frequency. Among them, the 13th order (651.1 Hz) of the system coupling mode is within the range of this howling and can be determined as the problem frequency.

3. Problem parts and improvement plans

Through the simulation results of MASTA software, the 13th order (651.1 Hz) of the system coupling mode is analyzed. As shown in Figure 8, the main problem component is that the high-speed gear of the intermediate shaft has too large dynamic response energy during the transmission process, accounting for more than 40% of the system dynamic response energy. According to the analysis of the problem components, it is found that the gear web stiffness of the high-speed gear is insufficient and the deformation is large, resulting in eccentric load in gear meshing and too high dynamic response of the housing at the output shaft. The simulation results are shown in Figure 9.

Figure 3 MASTA analysis model of an electric drive assembly

Table 1 Macro parameters of reducer transmission gear pair

Figure 4 High-speed gear pair transmission error (peak-to-peak difference: 0.839 mm)

Figure 5 Fourier transform of transmission error of high-speed gear pair (amplitude: 0.365 4 mm)

Figure 6 System coupling modes

Figure 7 System coupling mode results

Figure 8 13th order vibration results of system coupling mode

Figure 9 Dynamic response of the housing output shaft

As shown in Figure 9, the dynamic response amplitude of the housing at the front bearing of the output end reaches 1.744 mm at 651.1 Hz. In view of the above situation, considering increasing the thickness of the web of the high-speed gear, the thickness of 5 mm and 10 mm were taken for simulation comparison, and the dynamic response of the housing at the output end bearing seat was mainly analyzed. The results are shown in Table 2.

Combined with the data analysis, it can be seen that after increasing the thickness of the high-speed gear web, the stiffness of the system will be significantly improved. At the frequency of 651.1 Hz, the dynamic response of the housing at the front bearing of the output end is significantly improved. Among them, the thickness of the web increases by 5 mm, and the dynamic response of the housing at the front bearing of the output end decreases from 1.744 μm to 0.670 8 mm, which has met the standard requirements in the vehicle test standard.

Taking into account the vehicle's requirements for the NVH performance of the electric drive assembly and the manufacturing cost of assembly components, it was decided to increase the belly plate thickness of the high-speed large gear by 5 mm.

Table 2 Dynamic response of the front bearing seat at the output end of the housing (651.1 Hz)

Reducer assembly vehicle test

In order to verify the correctness of the above product optimization results and software analysis results, we conducted vehicle installation tests on the optimized assembly products and compared the test results with those before optimization. Under the vehicle WOT condition, the test results of the noise test of the reducer after subjective test optimization are significantly better than those before optimization. The near-field noise data in the vehicle is collected through the customer's LMS data acquisition front end, and the collected data is analyzed by the LMS Test.Lab data analysis software to perform noise order analysis on the near-field noise and verify the installation results.

Through verification, it can be seen that within the speed range of 2000-4000 r/min, the vibration is significantly reduced, which is consistent with the trend of the noise inside the vehicle. Above 4000 r/min, due to the inconsistent speed increase rate of the test vehicle, no comparison is made. From the start of the vehicle to the 2000 r/min range, the improvement effect is not obvious.

The comparison data of the two tests by the customer did not show any structural resonance at around 650 Hz in the 22nd-order noise curve. However, from the subjective evaluation of the noise inside the car, the howling problem in the speed range of 1,600 to 2,000 r/min has indeed been improved.

in conclusion

Based on the feedback from customers of an electric drive assembly project of our company, there is a problem of NVH howling. This paper uses MASTA software to perform finite element calculation and analysis on the electric drive system assembly and finds a corresponding solution. Although this solution solves the problem of excessive order noise raised by the customer, it does not solve the problem of low-frequency howling well. The test method and analysis method still need to be further optimized. We can still draw the following conclusions for reference by researchers in the same field:

1) The high integration of the electric drive assembly changes the coupling mode of the system, making the NVH problem of the assembly more complicated.

2) Adjustment of a certain component of the assembly will affect the coupled modes of the entire system.

3) The dynamic response of the system can be reduced by increasing the stiffness of certain components, solving some NVH noise problems caused by the transmission path.

4) Adjusting the system stiffness only reduces the NVH howling problem from the noise transmission path. The transmission error is the key to solving the NVH problem. Reducing the transmission error during the working process of the gear pair can reduce the vibration excitation source and fundamentally solve the NVH howling problem.